The present invention relates to a magnetic resonance imaging (MRI) system for observing magnetic resonance (MR) information of an object to be examined by utilizing a magnetic resonance (MR) phenomenon.
In an MRI system, a uniform static magnetic field is applied to a predetermined portion of an object to be examined, and a gradient field in a direction perpendicular to a surface of a specific slice portion including a portion of interest and an excitation pulse as a high-frequency (normally a radio frequency (RF)) magnetic field in a direction orthogonal to the static magnetic field are applied to the object to be examined and superposed on the static magnetic field. As a result, magnetic resonance (MR) is excited in specific nuclear spins of only the slice portion. In this system, a magnetic resonance (MR) signal generated after the magnetic field of the excitation pulse is deenergized is detected by the resonated nuclear spins, and the MR signal is processed to form image information. Note that in this system, when MR is to be excited and MR signals are to be acquired, gradient fields in a plurality of directions are applied to the slice portion as needed so that the acquired MR signals include position information. For example, in a case of two-dimensional Fourier transform, a phase-encoding gradient field in a first direction is used during excitation of MR, and a reading gradient field in a second direction is used during acquisition of the MR signals. The first and second directions are normally parallel to the selected slice surface and orthogonal to each other.
A conventional MRI system has the following drawbacks.
(1) When an MR image of a slice portion selected by the MRI system is to be formed, if humor, especially blood is flowed into the slice portion from the outside thereof, an artifact is caused in a phase-encoding direction. Such an artifact interferes with a diagnosis performed by observing an MR image.
(2) In order to obtain an MR image of a given local portion in a selected slice portion, a surface coil is used to receive an MR signal from only the local portion, thereby performing local imaging. However, a system using the surface coil is effective to form MR images of local portions on and near the surface of the object to be examined but not suitable for imaging a deep portion.
(3) When a diagnosis is made by extracting an MR signal of a given local portion in a selected slice portion and obtaining resonant frequency information on the basis of magnetic resonance (MR) spectroscopy, a topical magnetic resonance (TMR) system or the system using the surface coil is used. In the TMR system, only a local portion is selectively excited. In this system, in order to change a static magnetic field distribution a range of a current to be flowed through the coil or a position of an object to be examined must be changed, thereby complicating the system. Similar to the system described in item (2), the TMR system is effective to receive MR signals from portions on and near the surface of the object to be examined but cannot be used to image a deep portion.
An MRI system is proposed to solve the problems described in items (1), (2), and (3), wherein transverse and longitudinal magnetization components of regions adjacent to a region to be examined including a portion of interest are erased in advance, and then MR information of the region to be examined is obtained (Japanese Patent Application No: 61-190777).
According to the above system, when transverse and longitudinal magnetization components of two adjacent regions sandwiching a region to be examined are to be erased, the adjacent regions sandwiching the region to be examined are selectively excited using a .pi./2 pulse. In this case, in order to selectively excite the two adjacent regions, excitation pulses having different frequencies f1 and f2 (frequency bands are .DELTA.f1 and .DELTA.f2, respectively) sandwiching a frequency band corresponding to the region to be examined need be applied.
In order to generate the excitation pulses having different frequencies f1 and f2, an oscillator having a wide oscillation frequency band to be selected or two oscillators having different oscillation frequencies need be arranged.